A temperature gradient can be induced in an optical fiber containing a fiber Bragg grating (FBG) in order to change the characteristic spectral response of the grating. Such thermally adjustable devices show great potential for optical communication systems. It is known in the art how to impose a temperature change or gradient to a FBG for various purposes. Uniform heating along the length of the grating allows to shift the spectral response of the device, while a variable heating along said length allows to adjust the bandwidth and/or dispersion of the grating.
More particularly, U.S. Pat. No. 5,671,307 (LAUZON et al.) discloses the use of a temperature gradient to impose a chirp on a FBG. The temperature gradient is realised with a heat conductive substrate, such as a thin brass plate holding the portion of fiber containing the Bragg grating, and Peltier effect plates heating one end of the fiber and cooling the other. Lauzon suggests that the device might be used as a tuneable dispersion compensator for optical fiber communication links, but does not disclose any energy efficient embodiment of such a device.
European patent No. 0 867 736 (FARRIES et al.) also discloses a temperature-based device and method for wavelength and bandwidth tuning of an optical grating. This patent combines the application of a temperature gradient and a mechanical strain to modify the optical properties of the grating. This device requires gluing the fiber to a metal block along its entire length, which in practice is a technologically challenging operation.
U.S. Pat. No. 6,351,385 (AMUNDSON et al.) presents a method for enhancing the performance of thermally adjustable fiber grating devices by disposing them within a vessel that eliminates detrimental air currents around the fiber. This invention requires the application of a special resistive coating to the fiber itself for heating purposes. The coating thickness must be varied in a well controlled manner along the fiber in order to produce a desired temperature gradient.
As requirements of optical communication systems get more and more demanding, near ideal grating performance becomes critical in many applications. A practical method for efficiently applying an accurately controlled temperature gradient to a FBG that may be used in many applications is therefore needed.